Will all the assets be downloaded at startup?

In a real application (exe/apk) will all the assets be downloaded at startup or only when you go to the scene where the asset belongs?

You mean loaded
And as i remember that progress bar on your game start up screen is when everything is being loaded
That’s why we do not have loading screens between scenes

Where audio files was other story
I am most likely wrong here about it but more or less FROM WHAT I REMEMBER they are like books on the shelf they are there ready for you to read just not open
That’s why you need to preload them if they do not play when they should like there is a little bit of delay